1. Field of the Invention
The present invention relates to a megasonic cleaning apparatus for fabricating a semiconductor device, and more particularly, to a megasonic cleaning apparatus capable of obtaining a sufficient level of cleaning effect and preventing pattern lifting.
2. Description of the Related Art
With increases in the integration of semiconductor devices, the size of patterns and the interval between patterns become very small. Accordingly, wafer cleaning becomes important because contamination particles existing on the surface of a wafer could cause poor patterns in subsequent processes, for example, contamination particles which remain between the patterns formed of a conductive film could render the semiconductor device inoperative.
With a reduction of the size of a fine pattern to 1 μm or less, the permissible size of a contamination particle also decreases. Thus, it can be difficult to remove those small contamination particles through conventional cleaning methods because of the strong adhesion force upon a wafer.
Many methods for increasing the cleaning efficiency have been proposed to effectively provide a force capable of overcoming the adhesion force of material to be removed from a wafer.
One proposed method for removing contamination particles is to use a megasonic cleaning apparatus. The megasonic cleaning apparatus uses a stream of fluid, which is agitated at high frequency. The fluid used can be, for example, deionized water. The fluid stream is shot at the wafer to remove the contamination particles from the wafer surface and/or recessed surface of the wafer.
A megasonic cleaning apparatus basically includes a piezoelectric transducer for generating megasonic energy and an energy transfer means for transferring energy generated by the piezoelectric transducer to fluid medium. The energy transferred through the energy transfer means agitates the fluid medium so that the fluid medium has a high frequency. This agitated medium vibrates the wafer surface, such that contamination particles are physically separated from the wafer surface.
FIG. 1 shows a conventional megasonic cleaning apparatus having a quartz rod 20 to serve as an energy transfer means. The quartz rod 20 transfers energy generated by a piezoelectric transducer 10 to fluid medium, that is, cleaning solution 15, and has a cylindrical cleaning portion 20a and a tapered rear portion 20b. The energy transferred to the cleaning solution 15 through the quartz rod 20 agitates the cleaning solution 15 at a high frequency. The vibrated cleaning solution 15 vibrates a wafer 30 and removes contamination particles from the surface of the wafer 30. At this moment, the entire surface of the wafer 30 is cleaned by being rotated along the rotation of a spin plate 40 disposed under the wafer 30.
In the conventional megasonic cleaning apparatus, the amount of the energy applied from the quartz rod 20 to the wafer 30 through the cleaning solution 15 may not be uniform over the wafer 30 in the length direction of the quartz rod 20, that is, from the wafer edge E to the wafer centre C.
FIG. 2 shows a graph showing a variation of energy transferred to a wafer depending on the positions over the wafer according to the apparatus of FIG. 1. Referring to FIG. 2, the largest energy is applied to the wafer edge E, and the energy applied to the wafer 30 decreases toward the wafer centre C. That is, a conventional megasonic cleaning apparatus concentrates energy on the wafer edge E. This arises from the phenomenon that the largest energy comes out from the wafer edge E because the cleaning solution 15 on the wafer edge E contacts the quartz rod 20 first.
Accordingly, the amount of energy generated by the piezoelectric transducer 10 must increase to ensure that the wafer centre C to which a relatively small amount of energy is applied has a sufficient cleaning effect. However, if the energy amount increases for obtaining a sufficient cleaning effect, the wafer edge E receives excessive energy, leading to a serious problem that patterns P located around the wafer edge E are lifted. Thus, there is a need for a megasonic cleaning apparatus capable of delivering optimal energy throughout the wafer for effectively cleaning the wafer and preventing pattern lifting.